stdbuf: Code cleanup.

• Factor out path-setting code.
• Normalize usage().
• Remove unnecessary switch case.

Sponsored by: Klara, Inc.
Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D40799

(cherry picked from commit 78baa632092a87707481a47f47bfd45de9a072ef)

libstdbuf: Add proper SEE ALSO section.

Sponsored by: Klara, Inc.
Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D40800

(cherry picked from commit c09909ed889f4722e68b192e206dff1b03182edb)